The bogies of railway vehicles must be able to travel over distortions, that is to say be able to exhibit points of support on the ground which are not in the same plane, because the tracks can have defects, called “distortions”.
When the bogie is of the “rigid chassis” type, it is mainly the flexibility of the suspension between the axles and the bogie chassis which allows wheel load transfers to be limited when the vehicle travels over distortions in the track. When the bogie is of the “articulated chassis” type, such as, for example, the bogie described in document EP 0 409 128, it is the articulation of two half-chassis about a transverse axis which allows the wheel load transfers induced by distortions of the track to be limited.
However, a rigid chassis requires the presence of a primary suspension, that is to say of a suspension arranged between the axles and the bogie chassis, for absorbing distortions, which suspension is bulky and heavy. It is possible for an articulated chassis not to have primary suspensions, because deformations in the event of distortions are absorbed by the articulation of the chassis. This solution accordingly enables a weight saving to be made and is of greater interest economically.
The chassis of the bogie is either of the “exterior” type, when the side members are arranged outside the wheels, generally resting on axle boxes arranged outside the wheels, or, on the other hand, of the “interior” type when the side members are arranged inside the wheels, that is to say between the wheels, on axle boxes which are likewise arranged inside the wheels. An interior chassis enables the mass of the bogie to be lowered and its manufacturing costs to be reduced. Such a chassis also allows the brake callipers to be accommodated outside the chassis, which improves the accessibility of the callipers should they have to be removed.
The exterior chassis is more bulky and adds a considerable mass to the railway vehicle. However, it allows a larger and more powerful motor to be accommodated between the wheels because the axle boxes located outside the wheels free space between the wheels.